1. Field of the Invention
The present invention concerns bipolar electrodes for batteries and is particularly concerned with one of the components thereof, the conductive support/separator which carries on each of its faces the active material forming the positive electrode on one side and the negative electrode on the other side.
2. Discussion of the Background Information
The manufacture of batteries with bipolar electrodes, be they Ni-Cd, Pb-PbO.sub.2 or other couples, involves several problems some of which relates to the bipolar support/separator or its production. The bipolar support/separator is electronically conductive since, as indicated above, it carries the electrodes, and also fluid-tightly separates two successive elements.
A first problem relates to pressure differences that, during operation, may build up between different elements forming a so-called sealed battery. One of the possible causes of these pressure differences may be the existence of thermal gradients which inevitably occur between the elements adjacent the end plates which close the stack, and those adjacent the middle of the stack. These pressure differences may also be due to the electrodes situated in different elements having different electrical capacitances. Furthermore, if an excessive over-pressure is produced in one element, for safety reasons, this pressure should not exceed a predetermined threshold value. For such cases, one can envisage that each element includes a safety valve. However, since in the case of Ni-Cd, the thickness of one element is for example, between 2.5 and 3 mm, providing valves of very small dimensions is difficult, and therefore expensive.
A second problem pertains to the marginal parts of the bipolar support/separators, where the insulating structure separating on element from another is fixed. One of the means that can be provided to insure sealing between the elements consists of overmolding a structure made of a polymer, over the entire perimeter of each bipolar support/separator. This polymer must be an electrical insulator and must be stable under use conditions, i.e., in the range of temperatures and mechanical stresses it may be submitted to, and in the presence of the electrolyte used. Of course, sealing between two elements implies that there must be a good adherence between the polymer and the conductive support/separator, and also that the connection between the polymer structures of two neighboring elements must be fluid-tight. This connection may be provided by welding, for example, by locally melting the polymer.
It should, moreover be noted that the conductive support/separator, for evident reasons, because of the required light weight of the generator, is generally very thin, for example, 0.05 mm or 0.1 mm in the case of Ni-Cd. Damage to the battery can result from an elevation of the internal pressure in an element which, associated with a rise in the temperature, may produce a pressure on the internal edge of the polymer structure causing it to unstick from the conductive support/separator.